When do i get Ludens Echo on immoble mages?

You gets Ludens on mages that can generate the stacks quickly. Can you fire off 5 spells in like 8 seconds? Get ludens for sure.

Ziggs and Karth yes, Lux and Malz no.

Ziggs and Karth spam their spells a lot. Lux and Malz have fairly long CDs so it really kinda sucks on them. At least Malz has a dot going for him so he can possibly proc it one more time after they run.

after you get mana regen

Substitute Ludens for Zhonyas in your build when the enemy team doesn't have a lot of kill pressure on you. The extra damage and waveclear is nice for almost any mage, but only if you're sure you won't need the Zhonya's active.

Ludens on Lux and Ziggs is great. The poke damage u get is insane just make sure you keep moving between casts cause that adds stacks. I don't like getting it on Malzahar because he is so short range and you generally want more burst then poke. Idk about Karthus tho since i don't play him. In general Ludens is great after your mana item like athenes or morello since it does a lot of burst. That being said I wouldn't get it if your facing strong ad assassins like Zed or Talon until after you get hourglass.

When do u get luden's echo on any mage? When you need more dmg outside of your skills and/or are playing a champion that can stack up the passive quickly.

You can build ludens echo on any of these champions. It functions as a great snowball item. Just note that in longer teamfights, how quickly your champion can stack up the passive is an important factor.

-Malzahar: Never. You don't have an item slot for it, sorry to say. -Lux: Grail Sorcs (ludens/deathcap) zhonyas void staff (luden's/deathcap). Ludens first if you want more burst damage/are snowballing, deathcap first if you want stronger shields. -Ziggs: Grail sorce (ludens/zhonyas) (luden's/zhonya's) voidstaff deathcap. Get ludens first if there are no real dive threats. Zhonya's first if you'll die without it. -Karthus: Uh... Maybe something like sorcs roa zhonya deathcap void ludens would be acceptable but I'd rather have the seraphs. If you're ballsy af, you could do sorcs seraphs zhonya deathcap void ludens but O.O

Well Ludens is amazing on so there's that.

Don't get it on Malzahar.